1. Field of the Invention
This invention relates to a device for protecting lattice members in a crane boom and for protecting a wire rope, hoist line that extends near the lattice members from damage due to impact and abrasion resulting from the hoist line pounding, slapping and rubbing against the lattice members as the hoist line is alternately slackened and tensioned.
2. Description of the Prior Art
Lifting cranes are used in a variety of applications that involve a cyclical work period of alternately slackening and tensioning a hoist line. Such applications include handling a wood yard grapple, a scrap yard magnet, a clamshell bucket, a dragline bucket, a concrete bucket, a pile driver, a pile extractor, a demolition ball and various other crane operations that involve alternately lifting and lowering. Since the hoist line is slack at times and taut at other times during the work cycle, it acts like an enormous "violin string" and possesses tremendous energy which must be dissipated. The position of the hoist line relative to the boom changes rapidly and it frequently slaps, pounds, vibrates and rubs the lattice members within the boom. This results in damage to the lattice members and to the wire rope, hoist line. Such damage, unless corrected by repair or replacement, can cause boom failures and broken hoist lines.
Many attempts have been made at solving this problem. Rollers have been attached, at spaced intervals along the boom where contact is likely to occur, to provide an anti-friction support surface for the hoist line. Such rollers are expensive and heavy. Thus, the rollers used have often been spaced too far apart along the boom and have had a short transverse span so that complete protection was not provided. To supplement the rollers, heavy timbers have been added to the boom members to provide a wearing surface.
Due to the cost, weight and service life of protection systems utilizing rollers and timbers that have been installed by crane manufacturers, replacement after wear out is often neglected because such systems are considered to be unsatisfactory. In an attempt to solve the problem, field maintenance personnel have tied old tires or other material to the crane boom at likely points of wear. While such field systems provide about the same protection as those installed by crane manufacturers, they eventually create hazards to operating personnel due to the material coming loose and falling from the boom. Then, the field system is frequently abandoned and the exposed boom members are subjected to hoist line contact.